Linear power supplies use a transformer to step down a.c. line voltage. Because a.c. line voltage commonly varies widely with time, the transformer must be designed to provide a minimum acceptable output voltage when the line voltage is at its lowest anticipated value. A series pass regulator circuit is sometimes used to provide a variable resistance so that the voltage delivered by the power supply is kept at a constant value.
Such power supplies are inefficient in high power applications. Power dissipated in the series pass regulator is much higher than it would be if varying line voltage conditions could be ignored in designing the power supply.
One prior art attempt to eliminate this inefficiency was to use a three-winding ferro-resonant transformer in place of the usual step down transformer. The ferro-resonant transformer maintained a constant output voltage by producing a magnetic resonance at a particular line frequency. The transformer core could then partially saturate in response to line voltage increases, thus decreasing transformer efficiency and stabilizing the output voltage. However, ferro-resonant transformers suffered the disadvantage that the resonance occurred only for a very small frequency range. Actual power line frequencies may vary over wide ranges (48-66 Hz) for which the ferro-resonant transformer performed as poorly as a standard step down transformer. An additional problem was the high cost of ferro-resonant transformers as compared to standard transformers.
Another prior art attempt to reduce inefficiency was the use of a switching regulator. Two switches were alternately opened and closed to produce a high frequency square wave to be applied to the step down transformer. The stepped voltage was dependent upon the frequency of switching and upon the interval each switch remained closed. The efficiency of these devices was independent of line voltage or line frequency fluctuations. However, there were disadvantages to these devices. First, the strong electromagnetic fields created by the switching action could lead to conduction of energy back into the power line, producing interference with other electrically powered devices. Also, the voltage amplitude of the square wave could be as high as 180 volts, requiring costly shielding to protect service personnel from shock hazards. Finally, switching regulators added a factor of unreliability because component aging could change switching speeds. Degradation in switch timing could also lead to simultaneous closing of the switches, producing fire and explosion hazards from a short circuit across a high potential difference.